Acyl carrier protein (ACP) or acyl carrier proteins (ACPs) are a small acidic protein (8,800 Da) responsible for acyl group activation in fatty acid biosynthesis. The gene encoding ACP (acpP) has been cloned and overexpressed (Rawlings, M. and Cronan, J. E., Jr. (1992) J. Biol. Chem., 267, 5751–5754; Jones, A. L., et al. (1993) Biochem. Soc. Trans., 21, 202S) and the solution structure of ACP has been solved by NMR spectroscopy (Holak, T. et al. (1988) Eur. J. Biochem. 175:9–15). Homologs of E. coli ACP exist throughout nature in two forms; either as an integral domain of a much larger multifunctional enzyme (type I) or as a discrete protein capable of associating with several other enzymes constituting a multienzyme synthase complex (typeII). In these two forms, ACPs play central roles in a broad range of other biosynthetic pathways that depend on iterative acyl transfer steps, including polyketide (Shen, B., et al. (1992) J. Bacteriol. 174:3818–3821), non-ribosomal peptide (Baldwin, J. E., et al. (1991) J. Antibiot. 44:241–247), and depsipeptide biosynthesis (Rusnak, F., et al. (1991) Biochemistry 30:2916–2927) as well as in the transacylation of oligosaccharides (Geiger, O., et al. (1991) J. Bacteriol. 173:2872–2878) and proteins (Issartel, J. P., et al. (1991) Nature 351:759–761).
A definitive feature of ACP is the 4′-phosphopantetheine (4′-PP) prosthetic group (Majerus, P. W. et al. (1965) Proc. Natl. Acad. Sci. USA 53:410–417). 4′-PP is attached through a phosphodiester linkage to a conserved serine residue found in all ACPs. Acyl groups of the many substrates recognized by type I and type II ACPs are activated for acyl transfer through a thioester linkage to the terminal cysteamine thiol of the 4′-PP moiety. The β-alanyl and pantothenate portions of the 4′-PP structure are believed to serve as a tether between the phosphodiester-ACP linkage and the terminal thioester, suggesting that 4′-PP may function as a swinging arm, shuttling growing acyl chains between various active sites, e.g. as in the sequential addition of 11 amino acids by the 800 kDa cyclosporin synthetase (Lawen, A. and Zocher, R. (1990) J. Biol. Chem. 265:11355–11360).
Holo-ACP synthase (holo-ACPS) transfers the 4′-PP moiety from Coenzyme A (CoA) to Ser-36 of apo-ACP to produce holo-ACP and 3′,5′-ADP in a Mg2+ dependent reaction. The (acyl carrier synthase protein) ACPS from E. coli was partially purified 780-fold from crude extracts (Elovson, J. and Vagelos, P. R. (1968) J. Biol. Chem. 243:3603–3611), and the ACPS from spinach has been partially purified (Elhussein, S. A.,et al. (1988) Biochem. J. 252:39–45), but remarkably little has been shown about the mechanism or specificity of this post-translational phosphopantetheinylation process. A mutant of E. coli conditionally defective in the synthesis of holo-ACP has been identified and the mutant phenotype attributed to an altered holo-ACP synthase activity (Polacco, M. L. and Cronan, J. E., Jr. (1981) J. Biol. Chem. 256:5750–5754).